Efforts have been made for a long time to formulate ascorbic acid (vitamin C) in the cosmetic and dermatological fields, in various galenic forms, because of its numerous beneficial properties. In particular, ascorbic acid stimulates the synthesis of connective tissue, in particular of collagen, enhances the defenses of skin tissue against damage by environmental agents such as ultraviolet radiation and pollution, compensates for any vitamin E deficiency of the skin, depigments the skin and has an anti-free radical function. These last two properties make it an excellent candidate as a cosmetic or dermatological active agent for combating and/or preventing skin ageing.
Unfortunately, because of its chemical structure (alpha-ketolactone), ascorbic acid is very sensitive to certain environmental parameters such as light, oxygen, water (through its pH and through the presence of traces of metals). The rapid degradation of ascorbic acid formulations occurs in the presence of these conditions, which is counter to the desired results. In order to reduce and/or retard the degradation of ascorbic acid, several solutions have already been envisioned in the prior art.
One of such solutions consists in blocking the reactive site of ascorbic acid, namely the hydroxyl site, especially by esterification or etherification with phosphated, sulphated or alkylated compounds (see in U.S. Pat. No. 5,087,446, JP-A-69/115558 and JP-A-83/129892 all incorporated herein by reference). Although more stable than ascorbic acid, these ascorbic acid derivatives are unfortunately much less effective than ascorbic acid itself.
Another solution consists in formulating the ascorbic acid in an acidic gel having a pH of not more than 3.5 and containing a high quantity of glycol, as described in WO-A-90/12572 incorporated herein by reference. Because of the acidic pH used and the presence of a large quantity of glycol, that is to say of solvent, this gel is poorly tolerated by the skin, whose pH is about 5.5. A repeated application of this gel to the skin causes irritation.
Another solution consists in using ascorbic acid in powdered form and in dissolving it in water just before use. Because of the high acidity of the solution obtained, it is aggressive and risks causing substantial irritation of the skin.
Another solution uses the freeze-drying of an aqueous solution containing an ester of ascorbic acid and hyaluronic acid; the freeze-dried product obtained is dissolved in water just before use. This solution has the disadvantages of being complex and quite expensive as well as using an ascorbic acid derivative which is less effective than that of ascorbic acid itself. In addition, the solution obtained is difficult to apply to the skin; it runs down the face and/or the body and is difficult to handle.
FR-A-2,645,740, incorporated herein by reference, describes a composition containing an oily phase, an aqueous phase and a solid phase to be mixed immediately before use, the solid phase containing one or more active agents (especially vitamin A) and an emulsifier, and being placed in the cap of a bottle containing the two liquid phases. This packaging allows a long storage time of the composition before mixing the three phases by shaking. After mixing, however, the composition is stable for only a few hours. Furthermore, the mixture obtained is a coarse emulsion which is not homogeneous. This results in lower efficacy; indeed, the coarser the emulsion, the less satisfactory it is as a vehicle for an active agent. In addition, the solid dissolves poorly. This three-phase composition cannot be used for an active agent with an acidic functional group, the acidic active agent carrying the risk of breaking the emulsion, or even of preventing its formation.
Other methods of stabilizing ascorbic acid include its coating (see, e.g., the technique described in FR-A-1,600,826 incorporated herein by reference). This technique is, however, on the one hand expensive and can, on the other hand, impair the ascorbic acid, for example during heating.
A topical composition for the hair, containing a solid product comprising an acid and a carbonate or bicarbonate, which are capable of reacting together, and a fluid carrier, is known from EP-A-399,157 incorporated herein by reference. This document teaches the formation of foam and consequently the necessary presence of carbonate or bicarbonate intended to react with the acid and does not teach the use of the acid alone in order to avoid its decomposition by the fluid carrier. The problem is therefore different in this case as compared to the present invention, as is the solution proposed.